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Why did Prussia gain the Rhineland and Westphalia after the Congress of Vienna?

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Historical map showing the unification of Germany by Prussia in the late 19th century. The map highlights in green the Kingdom of Prussia as of 1866, and in yellow the additional territories that formed the German Empire by 1871-1918, encompassing most of north-central Europe. Neighboring countries such as France, Belgium, the Netherlands, Denmark, Sweden, Russia, Austria-Hungary, and Switzerland are labeled, with major regions and states within the empire clearly marked. — Prussia The unification of Germany by Prussia, which brought most of north-central Europe into one kingdom.

At the Congress of Vienna, the decision to grant Prussia control over the Rhineland and Westphalia was primarily a strategic move by the European powers to ensure regional security.12

Following the defeat of Napoleon, the victorious allies sought to establish a balance of power that would prevent future French aggression. Specifically, Viscount Castlereagh of Great Britain insisted that Prussia take on the role of a "defender of the western boundary of Germany". By positioning Prussia in these western territories, the Congress created a strong, militarized buffer state that could act as a buttress to the newly formed Kingdom of the Netherlands and effectively guard against potential incursions from France.3412

This territorial adjustment also served as compensation for Prussia, which had ceded the bulk of its Polish possessions to Russia in exchange for these western lands and a portion of Saxony.13